Motorized caulk guns which include a piston rod acting on the internal piston of a caulk cartridge are known. An example of such a gun is shown in U.S. Pat. No. 4,264,021, in which a piston rod extends through and is driven by a drive plate. The drive plate is further driven by a cam mounted on a gear, with the gear driven by the motor. Though this system provides for power operation, it suffers from the drawback that the rate of advance of the piston is inconsistent since the drive plate is necessarily out of contact with the cam surface at some time during each rotation of the gear on which the cam is mounted. In addition, the trigger mechanism functions only to control the motor, and does not otherwise control movement of the piston rod. As a result, no manual operation of the caulk gun is possible.